I use vim (Actually gvim on windows) as my main text editor. In my work flow I have to copy sentences to/from various external sources, therefore I use clipboard=unnamed to
I don't know if I misunderstand you but I tried what you are doing and I have no problem in doing that with the + drop-register.
My workflow:
Sadly when pasting the second time you have to explicitly paste from the + register. Therefore I would recommend a mapping for p/P instead of using clipboard=unnamed
nmap p "+p
Try using
:registers
to see the contents of the different registers.